1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, in particular, a method for manufacturing a semiconductor device which uses a reactive ion etching step in the formation of a fine pattern.
2. Description of the Related Art
Recently, attempts have been made to fabricate semiconductor devices of an improved characteristic and high integration density. In the achievement of a high integration density unit, a recent tendency is mainly toward using a microfabrication technique for forming an arbitrary pattern on a treating or etching layer with the use of a photoresist and stepper and selectively etching the layer.
For example, the submicron line dimension, such as a line width 0.7 .mu.m, has been used in the fabrication of high-tech products, such as 4 Mbit dynamic random access memory (DRAM). In the formation of a submicron pattern layer, a photoresist pattern (a mask for etching) has a sharp side wall so as to eliminate a finishing dimension error resulting from the side etching of the mask pattern at an etching step. Further, a reactive ion etching is used in etching a treating layer by directing a gaseous ion in a direction perpendicular to the major surface of the treating layer. The etching action progresses partly due to a chemical reaction of the reactive ion with that of the treating layer and partly due to the sputtering by an incident ion. The sputtering is performed not only on the side wall of the photoresist (a mask material) but also on the side wall of the treating layer. Since the side wall of the photoresist mask pattern is not sharp in the fabrication of a conventional semiconductor device, a sputtered foreign substance deposited on the side wall of the pattern was able to be removed or, if not removed, never projected above the top surface (major surface) level, posing no appreciable problem. In the reactive ion etching step using a sharp-sided pattern, however, a foreign substance deposited on the pattern side wall is not etched away, leaving it unremoved from the rest of the semiconductor body. The foreign substance is not removed by a subsequent oxide plasma ashing step made on the photoresist and is left as a foreign projection. The left foreign substance (sputtered foreign film) is also not removed in the subsequent formation of a layer during the manufacture of the semiconductor device and penetrates that layer.
Japanese Patent Publication 54-18973 proposes removing a deposition of a foreign film. This method comprises, subsequent to forming of a mask pattern on a treating layer, performing an etching step to remove a foreign film or substance o the side wall of the mask pattern or on a treating layer by a wet chemical etching method using an inorganic acid and performing a plasma ashing step to remove the mask pattern.
This method encounters a drawback in that the etching layer per se is adversely affected due to a local action of the etching to produce many defective products. This is probably due to the fact that, because of a progress of the chemical etching prior to the removal of the resist, an element such as chlorine present in small quantity in the resist as derived from an etching gas acts catalytically on the etching layer to progress the etching action there.
With a semiconductor device of high integration density, the formation of a fine pattern is an important requisite. For this reason, the major tendency is toward the use of the sharp-sided photoresist pattern and the microfabrication using the reactive ion etching method. In this case, a foreign projection is deposited on the side wall of the pattern at the time of such etching, causing a defective isolation and defective configuration and hence a malfunction in the semiconductor device. There is a growing demand for an effective method for the removal of such foreign film or projection.